Metal-environment interface

The effective use of metals as materials of construction must be based on an understanding of their physical, mechanical and chemical properties. These last, as pointed out earlier, cannot be divorced from the environmental conditions prevailing. Any fundamental approach to the phenomena of corrosion must therefore involve consideration of the structural features of the metal, the nature of the environment and the reactions that occur at the metal/environment interface. The more important factors involved may be summarised as follows ... 

Metal/environment interface—V ne cs of metal oxidation and dissolution, kinetics of reduction of species in solution nature and location of corrosion products him growth and him dissolution, etc. 

The enormous scope of the subject of corrosion follows from the definition which has been adopted in the present work. Corrosion will include all reactions at a metal/environment interface irrespective of whether the reaction is beneficial or detrimental to the metal concerned —no distinction is made between chemical or electropolishing of a metal in an acid and the adventitious deterioration of metal plant by acid attack. It follows, therefore, that a comprehensive work on the subject of corrosion should include an account of batteries, electrorefining, chemical machining, chemical and electrochemical polishing, etc. 

The basic mechanism for the instability of ultrapure metals was suggested by Wagner and Traud in a classic paper in 1938.1 The essence of their view is that for corrosion to occur, there need not exist spatially separated electron-sink and -source areas on the corroding metal. Hence, impurities or other heterogeneities on the surface are not essential for the occurrence of corrosion. The necessary and sufficient condition for corrosion is that the metal dissolution reaction and some electronation reaction proceed simultaneously at the metal/environment interface. For these two processes to take place simultaneously, it is necessary and sufficient that the corrosion potential be more positive than the equilibrium potential of the M, + + ne M reaction and more negative than the equilibrium potential of the electronation (cathodic) reaction A + ne — D involving electron acceptors contained in the electrolyte (Fig. 12.8). 

Anion Migration into Occluded Regions. Anions in the external environment, particularly chloride ions, will migrate into the occluded region as a consequence of the potential difference between the solution at the metal/environment interface in the pit and the solution at the external surface or, equivalently, in response to the increase in positive charge resulting from the increased cation concentration in the bottom of the pit. Chloride ions are known to stabilize the hydrolysis reactions and actually further lower the pH (Ref 19). If the increase in metal-ion concentration associated with the anodic current density at the pit inter-... 

Simple but pedagogically useful theories of electrode kinetics are presented in Chapter 3. This permits discussion of models for anodic and cathodic reactions at the metal/environment interface and for diffusion of species to and from the interface. Mathematical models of these theories lead to so-called kinetic parameters whose values govern the rate of the interface reaction. The range of values that these parameters can have and some of the variables that can influence the values are emphasized since these will relate to understanding the influence of such factors as surface conditions (roughness, corrosion product films, etc.), corrosion inhibitors and accelerators, and fluid velocity on corrosion rates. This chapter also introduces electrochemical measurements to determine values of the kinetic parameters. 

The thermodynamics of corrosion processes provides a tool to determine the theoretical tendency of metals to corrode. Thus, the role of corrosion thermodynamics is to determine the conditions under which the corrosion occurs and how to prevent corrosion at the metal/environment interface. Thermodynamics, however, cannot be used to predict the rate at which the corrosion reaction will proceed [1—6]. The corrosion rate must be estimated by Faraday s law and is controlled by the kinetics of the electrochemical reaction. 

MOLECULAR MODELING OF STRUCTURE AND REACTIVITY AT THE METAL/ENVIRONMENT INTERFACE... 

First-principles thermodynamic calculations can be useful in this context by comparing the relative strength of chemisorption for different ions at the metal/environment interface and delineating the relevant thermodynamic conditions. First-principles thermodynamics involves the extrapolation of internal energies determined at OK via electron structure calculation to finite temperature free energies through the incorporation of vibrational, rotational, and translational enthalpic and entropic contributions as well as configurational effects. 

The metal/environment interface poses a fascinatingly complex theoretical problem, which, as illustrated in the aforementioned case studies, has only Just begun to be interrogated using advanced atomistic modeling and simulation techniques. The context of corrosion provides a... 

Capturing not only details of the metal/environment interface but also the metal/oxide and oxide/environment interfaces... 

Because the corrosion behavior of metals is governed by complex interactions involving many parameters, it manifests itself in numerous often unanticipated forms. The corrosion resistance of a given metal is not an intrinsic property of that metal, but a systems property. The same metal may rapidly corrode in a certain environment while under different conditions it is stable. From a more fundamental point of view, the corrosion resistance of metals is essentially determined by the reactivity of the metal-environment interface. Therefore the chemical and structural characterization of surfaces and interfaces (cf. Chapter 3) and the smdy of their electrochemical behavior in a given environment (cf. Chapter 4) are important aspects of corrosion science. 

Corrosion as a Chemical Reaction at a Metal/Environment interface... 

When microorganisms are involved in the corrosion of metals, the situation is more complicated than for an abiotic environment, because microorganisms not only modify the near-surface environmental chemistry via microbial metabolism but also may interfere with the electrochemical processes occurring at the metal-environment interface. Many industrial systems are likely to contain various structures where MIC and biofouling may cause serious problems open or closed cooling systems, water injection lines, storage tanks, residual water treatment systems, filtration systems, different types of pipes, reverse osmosis membranes, potable water distribution systems and most areas where water can stagnate. 

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